Contact lens wearers often dispose of the products by washing them down the drain or flushing them down the toilet — and that’s bad news for the environment, new research suggests.

Lenses that are washed down the drain ultimately end up in wastewater treatment plants, according to a team that presented its findings at the 256th National Meeting & Exposition of the American Chemical Society. The scientists estimate that anywhere from six to 10 metric tons of plastic lenses end up in wastewater in the U.S. alone each year.

Contacts tend to be denser than water, which means they sink, and this could ultimately pose a threat to aquatic life, especially bottom feeders that may ingest the contacts, according to a press release from the American Chemical Society.

“We found that 15 to 20 percent of contact wearers are flushing the lenses down the sink or toilet,” said Charlie Rolsky, a Ph.D. student who presented the work. “This is a pretty large number, considering roughly 45 million people in the U.S. alone wear contact lenses.”

Rolsky, Rolf Halden, and Varun Kelkar are at the Biodesign Institute’s Center for Environmental Health Engineering at Arizona State University.

To help address the fate of contact lenses during treatment, the researchers exposed five polymers found in many manufacturers’ contact lenses to anaerobic and aerobic microorganisms present at wastewater treatment plants for varying times and performed Raman spectroscopy to analyze them.

“We found that there were noticeable changes in the bonds of the contact lenses after long-term treatment with the plant’s microbes,” Kelkar said.

The team concluded that microbes in the wastewater treatment facility actually altered the surface of the contact lenses, weakening the bonds in the plastic polymers.

“When the plastic loses some of its structural strength, it will break down physically,” Kelkar said. “This leads to smaller plastic particles which would ultimately lead to the formation of microplastics.”

Aquatic organisms can mistake microplastics for food and since plastics are indigestible, this dramatically affects the marine animals’ digestive system. The animals are part of a long food chain. Some eventually find their way to the human food supply, which could lead to unwanted human exposures to plastic contaminants and pollutants that stick to the surfaces of the plastics.

The team hopes the industry will take note of the research and, at a minimum, provide a label on the packaging describing how to properly dispose of contact lenses, which is by placing them with other solid waste.

“Ultimately, we hope that manufacturers will conduct more research on how the lenses impact aquatic life and how fast the lenses degrade in a marine environment,”